Multiple Glazing

units. Many manufacturers of glass and windows utilize one or more of the following.

a. Interior glazing sheets of very thin, durable, highly transparent plastic film. These add airspaces while adding little thickness and almost no weight.

b. Low-conductivity gas fills instead of dry air between the panes, using gases such as argon or sulfur hexa-fluoride.

c. Low-emissivity (Low-E) coatings on inside surfaces of glass. These coatings are usually formulated for use in cold climates, to be highly transparent to solar wavelengths of light and heat, but to reflect the longer infrared wavelengths that are characteristic of heat radiated from the interior of a building. For use in climates and buildings where summer cooling is the primary problem, other formulas arc used to produce coatings that reflect most solar heat.

d. Edge spacer details between the sheets of glass that are less conductive of heat than the conventional metal spline.

At this writing, the most thermally efficient multiple glazing units, by utilizing a combination of these devices, achieve an insulating value of R-8 (56)—about 40% as good as that of a well-insulated wall. For more information on the thermal resistance of various types of glazing, see manufacturers' literature for windows and glazing. When we are evaluating alternative types of windows for their thermal resistances, it is important that we compare test values for entire window units, rather than for center of glazing. Window unit values include the effects of the glass, edge seal, and frame, while centcr-of-glazing values relate only to the glass itself. ■

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